Final Senior Presentation
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Transcript of Final Senior Presentation
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The Genetics of Adaptation: A Reassessment
H. Allen Orr and Terry A. Coyne
B11: Anthony Murphy
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The Goal Review the theory and data supporting the
claim that adaptation results from accumulation of many alleles of small effect
Show that there is no real solid data to back up this claim
With the introduction of molecular approaches, there might be the ability to resolve this problem in the future
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Micro-mutationismOriginated with Charles
DarwinDarwin and followers
believed evolution of adaptive systems happened in substitution of many genes of small effect
Darwin wrote “We have many slight difference which may be called individual differences…..these differences are important for us, as they afford materials for natural selection”
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Macro-Mutationalism Originated with T.H. Huxley, a
colleague of Charles Darwin Huxley and followers believed
that adaptive evolution takes place in large steps with no intermediate steps
Huxley wrote: “Mr. Darwin’s position might..have been stronger than it is if he had not embarrassed himself with the aphorism, ‘Natura non facit saltum’…We believe that Nature does make jumps now and then, and recognition of the fact is of no small importance in disposing of many minor objections to the doctrine of transmutation.”
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R.A FisherChampion of the Micro-
mutationism Theory Argued that adaptation
involves “conformity of parts” of the system
Large Random Change would cause “worsening of function”
He believed large mutations have a very small changes of being favorable
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Fisher’s Microscope AnalogyConsider a
microscope a system for adaptation
A large change will almost always worsen the focus in an ideal microscope
However, small changes are less likely to harm the focus and might improve it
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Fisher’s Sphere ExampleArrangement of parts
giving the highest fitness is the center of the sphere
A species’ present position can be represented by a part of the sphere surface
Any displacement inside the sphere improves adaptation
Any displacement outside the sphere worsens adaptation
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Why Fisher is Incorrect Fisher shows only that small
mutations are more likely to be favorable, not that they are more likely to be substituted
The substitution rate of a class does not depend only on the chance of it being advantageous, but also on the mutation rate and fixation rate (According to Kimura)
Fisher’s argument does not apply to cases in which an adaptive landscape has more than one single peak
Fisher assumes that all mutations have a similar effect on fitness no matter how they interact in the body
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Lande (1983)Concluded that, unless selection is very
strong and persistent, adaptation will usually result from substitution of micromutations.
Explains why artificial selection or selection in disturbed populations usually fix mutations of large affect while natural selection fixes polygenes
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Problems with Lande’s ModelIt is considered, by the authors, to be an
improvement over Fisher, but still has problems
It is not clear that evolution in nature usually involves selection toward an ideal form
Lande’s theory also may have features that predispose it against the fixation of large mutations
However, it should be pointed out that no model can answer the question of relative importance between major or minor genes.
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Frequency of Major and Minor MutationsThe rate of substitution of major genes
depends on how often they are produced by mutation
It is also said that mutations with small effects arise more often than those with major effects
There are many individuals who support the claim that mutations affecting viability
Only one (Gregory) investigated whether the same is true for genes affecting morphology
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Gregory (1965)Compared the phenotypes of peanut plants
descended from seeds irradiated by X Ray and control plants
Concluded that mutations of large phenotypic effect are far rarer than small effect mutations
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Problems with GregoryNo evidence that the mutations were genetic, Assessment of effects was not the same
between the two experimental variablesThe experiment was bias against the recovery
of large mutations
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Genetic Analysis of AdaptationArtificial Selection- Selection for traits in
breedingDisturbed Populations in Nature- Pioneer
Species, Resistance to environment, Arms race
Adaptations in Natural Populations- Batesen and Mullerian Mimicry
Differences among Species and Subspecies- Genetic Difference between species of organisms
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Future Research There are only a few genetic analyses of
species differences that are clear adaptations While some adaptations are based on genes
of small effect, sometimes major genes are involved
Traditional approaches can’t answer the central question
Must use genetic factors such as linkage and chromosome analysis
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ConclusionThey found little support that adaptation
always involves genes of small effectIt has been found that major gene effects are
not as rare as they were once thoughtThere is a need in the near future to perform
more genetic studies of adaptation